1. Field of the Invention
The present invention relates to a method for driving a projection cathode ray tube (CRT), and more particularly, to a method for driving a projection CRT by which an image of high brightness displayed on a screen is magnified and projected through a lens unit.
2. Description of the Related Art
In a typical projection CRT, since an image having high brightness should be displayed on a screen, a threshold voltage of the screen is relatively high. Here, the threshold voltage of a screen means a boundary voltage which determines emission of the screen. The threshold voltage of a screen is also referred to as a cutoff voltage of a screen.
In a laser projection CRT, a laser pixel array is attached on a disk formed of sapphire material. Each laser pixel is provided with three mono crystals respectively generating red (R), green (G), and blue (B) laser beams based on the energy of an electronic beam, and a pair of reflection mirrors facing each other. The energy needed to generate a laser beam in each mono crystal is higher than that needed to generate light from a fluorescent material of a common CRT. In other words, the threshold voltage of a screen of a laser projection CRT is higher than that of a screen of a common CRT. Of course, the threshold voltage of an electron gun of a laser projection CRT is the same as that of an electron gun of a common CRT. Here, the threshold voltage of an electron gun meals the voltage needed to generate an electron beam from an electron gun. The threshold voltage of the electron gun is referred to as a cutoff voltage of an electron gun.
As shown in the above, there is a wide gap between the threshold voltages of the screen and the electron beam in a projection CRT. However, in an image driving signal for driving the electron gun, the voltages of an alternating-current component should be determined according to the threshold voltage of the screen. That is, the voltage of a direct-current component is set lower than the threshold voltage of the screen and higher than the threshold voltage of the electron gun. FIG. 1 shows a waveform of an electronic beam from the electron gun. Referring to the drawing, voltages of an alternating current component of the image driving signal should be set based on the threshold voltage VSCN of the screen. Accordingly, in view of the electron gun, the voltage VOVR difference between the threshold voltage VSCN of the screen and the threshold voltage VGUN of the electron gun is applied excessively. The area of FIG. 1 corresponds to the amount of the excessive energy due to the voltage difference VOVR. Since the voltage of a direct current component of the image driving signal is set higher than the threshold voltage VGUN, the electron gun continuously emits electron beams to the screen. Thus, the screen and the electron gun are easily deteriorated and their lives are shortened. Also, since the temperature of the screen increases relatively, power required for cooling the screen is consumed more. Furthermore, contrast of the screen displayed is lowered relatively.
To solve the above problem, it is an objective of the present invention to provide a method for driving a projection CRT by which unnecessary electron beams are not emitted from the electron gun.
Accordingly, to achieve the above objective, there is provided a method for driving a projection CRT by which an image of high brightness is displayed on a screen as an electron beam emitted from an electron gun is scanned according to an image driving signal and the image is magnified and projected through a lens unit, in which voltage of a direct current component of the image driving signal is lower than threshold voltage of the screen for scan time and also lower than threshold voltage of the electron gun for retrace time.